This invention relates to the production of extruded thermoplastic films, and more particularly to an improved process for preparing films from polyesters and laminates employing these films.
The number of uses for extruded plastic films in general and polyesters in particular, has increased steadily in recent years. Their uses in food packaging alone have accounted for much of the increased volume of polymer films used commercially. Films made of polyester materials are particularly useful in food packaging due to their good moisture barrier properties and their freedom from any harmful residues which may pass into the food materials. Polyesters films have been particularly useful in packaging frozen foods, and laminates of polyesters on paperboard substances are very suitable for forming rigid packaging structures which may be employed for transporting and storage of the foods as well as for use as cooking containers in microwave ovens.
There are essentially two methods for preparing films made of polyester resins. In one, a stream of molten polyester film-forming material is passed through an elongated sheeting die which has an elongated slit die opening. Upon exiting the die, the material is drawn from the die opening at a rate greater than the rate of extrusion. The film, drawn in this manner, it thinned out to a thickness suitable for the intended utility and can be cooled and rolled or coated onto a substance prior to cooling.
In the other process for preparing polyester films, a circular die having an annular die opening is employed. The molten polymer is passed from the annular opening to form a tubular extrudate in which a gas is held under pressure to stretch the film in the direction transverse to the machine direction. Stretching in the machine direction can also be obtained by virtue of nip rollers which collapse the film and draw it from the die opening at a rate greater than the rate of extrusion.
Both of these extrusion procedures have their known advantages as well as limitations. When a particular set of conditions dictates the use extrusion in the form of a flat film from an elongated sheeting die, there has always been the problem with polymers in general, and polyesters in particular, that the film tends to neck-in and fluctuate to an undesirable degree as it is drawn from the die opening. This neck-in causes a bead to form at the edges of the film and also causes the edge of the material to weave or otherwise become uneven. This problem is especially pronounced with the extrusion of polyester films. Because there are very few applications where a necked-in film can be used as is without trimming off the undesirable portions along the edges, there is a great deal of wastage involved not only in the polymer employed but, also any substrate to which it is applied to those cases where it is not practical to trim the film prior to laminating it to the substrate.
A number of procedures have been proposed to obviate the problem of heavy or uneven edges on extruded plastic materials in the form of films. One such proposal involves metering the extrudate with deckle rods, adapted to control the beading, disposed in the extrusion die. This is discussed, for example, in U.S. Pat. No. 2,982,995.
In another proposal for solving this problem, U.S. Pat. No. 3,356,556 suggests splitting the film longitudinally by means of at least one wire disposed in the path of the film prior to laminating the film to a suitable substrate. By maintaining the width of the film strip to less than about 1.5 inches, the problem of beading at the edge was not noticeable. However, this particular practice is not suitable for the formation of wider films, such as those required for use in food trays suitable for microwave heating or the like.
In another proposal for controlling the neck-in problem attempts have been made to provide means for holding either the inherently-formed or an intentionally-enlarged bead area to restrain the normal tendency of the film to decrease in width. Such means are in the nature of air jets which cool the film below the adhesion point. However, this procedure has not been as successful as would be desired in view of the high capital and operating expenses required. Thus, there remains a need to provide an improved process for extruding polyester films to avoid these problems.